The present invention relates to a device isolation method for a semiconductor device and, more particularly, to device isolation method for a semiconductor device for intentionally forming a void in a pad oxide layer thickened by a bird's beak punch through phenomenon resulting from use of a device isolation method using a local oxidation of silicon (LOCOS) process.
Along with the recent trend toward higher integration of semiconductor devices, the research and development of a device isolation technology (micro-technology) has been actively advanced. The formation of a device isolation region is an initial step in the semiconductor manufacturing process, and it determines the size of an active region and defines the process margin of all subsequent manufacturing process steps. Isolation has been achieved by the LOCOS method.
FIGS. 1A-1D show a method for forming a field oxide layer by a conventional LOCOS method.
As shown in FIG. 1A, after pad oxide layer 12 is formed on semiconductor substrate 10, nitride layer 14 is formed thereon.
As shown in FIG. 1B, photoresist is coated and patterned on nitride layer 14 to form photoresist pattern 16. Next, impurities of the same conductive type as the substrate are ion-implanted on the whole surface of the substrate, to form a channel stop region.
As shown in FIG. 1C, after nitride layer 14 is etched using photoresist pattern 16 as an etching mask and the photoresist pattern is removed, a thermal oxidation process is performed such that field oxide layer 18 is formed in the device isolation region of the substrate. At this time, channel stop region 20 is formed below field oxide layer 18.
As shown in FIG. 1D, device isolation is realized by removing nitride layer 14 and pad oxide layer 12.
It is well known, however, that in the LOCOS method serious problem results from formation of the bird's beak structure. The bird's beak structure occurs, because oxygen permeates to the side surface of the pad oxide layer under the nitride layer used as an oxidation prevention mask and the silicon under the nitride layer becomes oxidized. Further, in higher integration semiconductor devices, the cell interval in the active region becomes smaller. In the conventional LOCOS technology, as described above, it is impossible to prevent the bird's beak, punch-through, phenomenon by which adjacently formed bird's beaks make contact with each other under the nitride layer. If the cells are overlapped, this above phenomenon is especially serious at the end of the cells where the bird's beaks extend in three directions.
In the "P" end of an overlapped cell,shown in FIGS. 2 and 3A-3C, the bird's beaks contact each other as a result of bird's beak punch-through phenomenon and the pad oxide layer 18 located under nitride layer 14 is thickened (FIGS. 3A and 3C). The bird's beak phenomenon is not serious, in the central portion of the cell (FIG. 3B).
When the bird's beak punch-through phenomenon is generated, the definition of the active region becomes virtually impossible. Also, to form the active region, the oxide layer thickened by the bird's beak punch-through phenomenon must be removed. To remove the thickened oxide layer, an over-etching process needs to be performed. Here, however, due to the over-etching performed to the field oxide layer as well as to the thickened oxide film, element characteristics deteriorate or the actual device isolation becomes impossible (see FIG. 8A).